1. Field of the Invention
The present invention relates to a noise source and vector network analyzer (VNA) used for noise figure measurement.
2. Description of the Related Art
A variety of configurations of test equipment are available for measuring noise parameters of a device under test (DUT). The accuracy of noise measurements is particularly important for components intended to operate at RF/microwave frequencies, since lower-frequency components usually can meet noise specifications.
The noise parameters of a component are usually specified in terms of noise factor F. The noise factor of a device is defined as the ratio of the signal-to-noise ratio available from the device output to the signal-to-noise ratio delivered to the input of the device, at a standard reference temperature of 290K. Noise factor is often expressed in dB, at which time it is sometimes referred to as a noise figure (NF).
Conventional components used for measuring NF include a noise source, a receiver, and an S-parameter measuring apparatus such as a VNA. The noise source and receiver can be manufactured as components separate from the VNA as illustrated in FIG. 1, or the noise source and receiver can be included permanently within the VNA, as illustrated in FIG. 2.
FIG. 1 shows the noise source and receiver provided separate from the VNA for connection to measure the noise figure of a DUT. The VNA is connected separately to the components for calibration when desired. The noise source typically includes a noise diode connected to a power supply. The noise source provides noise over a wide bandwidth to one port of a DUT, while the other port is connected to a receiver. The receiver downconverts the noise source signal from the DUT to an intermediate frequency (IF) range, and the IF signal is processed to provide an indication of power level enabling the noise figure to be determined over the frequency range.
The noise source and receiver shown in FIG. 1 can be provided together in a single noise module for measuring noise figure. Such a noise module is disclosed in U.S. Pat. No. 5,191,294, The noise module described in U.S. Pat. No. 5,191,294 includes ports for connection of both a DUT and a VNA enabling use of the VNA to calibrate components for improved noise figure measurements without errors potentially resulting when a VNA is connected and disconnected during measurements.
Components for measurement of noise figure can also be included internally to some VNAs, as shown in FIG. 2. As shown, the noise source in the VNA is connectable by a switch A through a reflectometer A to the measurement port A of the VNA. The switch A normally connects the signal source of the VNA through the reflectometer A to the test port A for standard VNA measurements and is switched to the noise source when noise figure measurements are desired. Similarly, a receiver B is connectable by a switch B to the test port B of the VNA. The switch B then normally connects to the signal source for standard VNA measurements and is switched to the receiver when noise figure measurements are desired. The receiver is normally included in the VNA for downconverting and processing signals received from reflectometers A and B, and is further configured to downconvert and process the signal from the noise source as provided through the DUT when the switches A and B are connected for noise figure measurements.
While noise figure measurements have been performed for years with a wide variety of instruments, the instruments have lacked flexibility. Usually the noise source must be attached directly to the DUT for all measurements, or the noise source is provided internally in an instrument for all measurements. The user, thus, has little choice with respect to test setup or traceability, or the ability to easily change to using a noise source with a more appropriate ENR (excess noise ratio) for the DUT.
Devices for measuring noise figure also do not allow a selection of frequency bandwidths for measurements which may limit the types of DUTs that can be measured using one test device. The measurement bandwidth being fixed for a given test system may also lead to either inordinately long test times or to inaccurate results.
In accordance with the present invention, a configuration of test equipment is provided to allow user selection of different types of noise sources with different ENRs which can be connected either directly to a DUT, or through a VNA to the DUT to make noise figure measurements more flexible. The test equipment also includes a receiver with two IF signal processing paths to generate wide and narrow bandwidth power measurements to provide further flexibility.
Direct connection of the noise source to the DUT leads to the simplest measurement of noise figure and maintains tracability. Alternatively the noise source may be connected to the rear panel of the VNA with the noise source signal routed through a switch and other hardware to a first VNA test port where a DUT terminal is connected for testing. This approach, while one level less traceable than a direct connection of the noise source to the DUT, may lead to more convenient measurements since the DUT may remain connected to the ports for other measurements. The indirect connection approach may also add to more accurate results since, assuming the VNA has characterized the signal path, the VNA can make corrections to account for DUT mismatches, and to provide more accurate gain measurements.
The output of the DUT feeds a second VNA test port where the signal will pass through switches to a receiver for indicating power level. The receiver includes a downconverter which generates an IF signal which is provided to one of two paths for wideband and narrowband power measurements. The wideband IF path is provided for fastest measurements, but may be inappropriate for some narrowband DUTS. While numerical correction is available for moderately narrowband DUTs, this does not cover all scenarios. For this reason a narrowband IF path is made available for measuring the noise figure of very narrowband devices such as devices operating over less than a 1 MHz bandwidth.
Thus, in accordance with the present invention a dual IF scheme and a switchable noise source are incorporated together to:
(a) Enhance measurement flexibility by allowing a user to choose between the importance of traceability vs. fixturing simplicity and/or potential accuracy enhancements by selecting a noise source position;
(b) Allow a choice of measurement bandwidths enabling a wider range of DUTs to be measured;
(c) Enable access to the signal path so a user can recharacterize the path and improve measurement accuracy using a VNA;
(d) Enhance measurement flexibility by allowing a user to easily change the noise source to one that is more appropriate for the testing of a given DUT; and
(e) Keep the noise source external to the measurement system so that the noise source is not subject to the internal temperature rise within the measurement system, and the measurements maintain a higher level of accuracy.